A configuration that comprises a DC power circuit including a DC/DC converter, a DC/AC converter and a starting circuit is known for use as a lighting circuit for discharge lamps, such as metal halide lamps, that may be used as a light source for a vehicle such as an automobile. For example, a DC voltage supplied by a battery maybe converted into a desired voltage by the DC power circuit. Then the desired voltage is converted into an AC signal and is provided by the DC/AC converter at the following stage Thereafter, the AC output may be supplied, with a superimposed start signal, to the discharge lamp (see, for example, Japanese patent document JP-A-7-142182).
In a lighting control of a discharge lamp, a voltage (hereinafter referred to as an “OCV”) that is to be provided at an unloaded time, preceding the lighting of the discharge lamp (i.e., while the discharge lamp is turned off), is controlled. After the discharge lamp is turned on, following the reception of the start signal, the discharge lamp is shifted to a steady lighted state as transient input power is gradually reduced.
A switching regulator employing a transformer, for example, is used for the DC power circuit. A full-bridge circuit employing pairs of switching devices, for example, is used for the DC/AC converter.
With a conventional lighting circuit, circuit size and cost problems may be encountered. For example, a transformer used for the DC power circuit and a transformer that constitutes the starting circuit are both required, or the number of switching devices used for the DC/AC converter is increased.
When a discharge lamp is employed, for example, as an automobile light source, a discharge lamp lighting circuit must be arranged within a limited space (e.g., a lighting circuit unit must be accommodated inside a lamp).
The circuit size is increased for an arrangement in which voltage transformation is performed at two steps (DC voltage conversion and DC/AC conversion) and the circuit is not appropriate for downsizing. As a countermeasure for this, an arrangement is proposed in which an output raised at one step through voltage conversion by the DC/AC converter is supplied to a discharge lamp. As an example arrangement, a resonant voltage is raised by employing one transformer and a resonance circuit, and subsequently is supplied to a discharge lamp. Problems in this case are that discrepancies in the characteristics of parts, such as a transformer and a capacitor, are tolerated to a degree to maintain the lighting function, and that the discharge lamp, after being activated, is steadily and quickly shifted to a stable lighted state. When the discharge lamp is employed as an automobile light source, these conditions are required in order to ensure satisfactory safety for night-time running.